1. Field of the Invention
The present invention relates to a document reading unit that includes a document support plate, used to support a conveyed document, that is positioned so as to cover an image sensor used for the reading of a document that is conveyed, a scanner and an image forming apparatus.
2. Related Background Art
Conventionally known is a so-called document moving scanner that includes: a conveying unit, for conveying a document; and a document reading unit that, to read a document, is equipped with an image sensor and is located along the conveying path of the conveying unit.
A configuration of this type of scanner is disclosed wherein the image sensor of a document reading unit has a first array and a second array of multiple line sensors arranged widthwise and perpendicular to the direction in which a document is conveyed (see, for example, Japanese Patent Laid-Open Publication No. Sho 60-31357 (FIG. 1(a))).
The document reading unit of such a conventional scanner includes: an image sensor, equipped with multiple line sensors; a box-shaped support base, within which the image sensor is stored and mounted; and a contact glass, which supports a document conveyed to the image sensor by a conveying unit. An opening through which the individual line sensors face the document is formed in the support base. The contact glass is formed as a flat rectangular plate, and is arranged at the opening in the support base so as to cover the image sensor. This contact glass prevents dust from entering the image sensor from the outside, and ensures that the image sensor performs a reliable reading operation.
The contact glass of the document moving scanner must be attached, at the opening in the support base, so that in the document conveying area there is no protruding portion that can abut upon a document that is being conveyed, and so that the conveying of a document by the conveying unit is not interrupted.
Furthermore, the distance between the image sensor and the contact glass must be adjusted so that an accurate focal distance can be obtained for the individual line sensors of the image sensor in the direction perpendicular to the document sliding face of the contact glass. Therefore, in the configuration of the conventional document reading unit, spacers are located between the contact glass and the image sensor, and by employing urging members, such as springs, the image sensor is pressed against the spacers that, in turn, are pressed against the contact glass. Either this, or the image sensor is arranged on the support base and an adjustment mechanism is provided for adjusting the position of the image sensor relative to the contact glass.
In order to satisfy the above described requirement, a fixing member is fixed and secured to the document sliding face of the contact glass. The fixing member positioned outside the document conveying area of the contact glass, and the short sides of the contact glass which are parallel to the direction in which a document is conveyed. One end of the fixing member engages the document sliding face of the contact lens, and screws secure the other end to the support base.
Further, at the opening edge of the support base, a glass support portion, which provides widthwise support for the external edge of the contact glass, is formed perpendicular to the direction in which documents are conveyed. Double-sided adhesive tape is attached to the contact glass longitudinally, that is, widthwise relative to the direction travelled by a document, so that the double-sided adhesive tape can secure the contact glass to the glass support portion of the support base.
For the conventional scanner described above, the document reading unit must be disassembled when a defect is found in the document reading unit during the manufacturing process, or when a failure occurs in the image sensor.
Further, for a conventional document reading unit, when a comparatively large contact glass, one that extends 50 to 60 mm in the direction in which documents are conveyed and is about 1 m wide, is to be separated from the glass support portion of the support base, the strength of the adhesion provided by the double-sided adhesive tape makes it very difficult to peel the outer edge of the contact glass away from the glass support portion of the support base.